GEOMEMBRANE LINERS FOR RESOURCE AND ENVIRONMENTAL PROTECTION: ENSURING LONG TERM PERFORMANCE

GEOMEMBRANE LINERS FOR RESOURCE AND ENVIRONMENTAL PROTECTION: ENSURING LONG TERM PERFORMANCE Ian D. Peggs I-CORP INTERNATIONAL Ocean Ridge, Florida, USA INTRODUCTION Geomembranes have rapidly become accepted as standard components of geosynthetic lining systems in landfills, ponds, canals, dams, reservoirs, heap leach pads, wastewater treatment plants, fish farms, and agricultural facilities. They isolate leachates generated by waste (municipal, industrial, and hazardous) to prevent groundwater contamination, they contain valuable product to prevent its loss into the ground, they prevent rainwater from becoming leachate, they prevent evaporation and contamination of drinking water, and they make dams waterproof. And they make very cost-effective, efficient, space-saving barrier systems. But their success has generated conventions and concepts that are contributing to an increasing number of performance problems, or failures. These failures are still a small percentage of the total industry but they receive much attention that can make some engineers, regulators, and facility owners uncomfortable in using geomembranes. However, in a new industry they are unavoidable, but fortunately they provide a great learning opportunity. The real disaster is if we do not learn from these failures and if we do not put what we learn into practice. The objective of this paper is to identify the problems that are occurring so that durable, properly performing geomembrane lining systems can be constructed that will benefit us all. THE PHILOSOPHICAL PROBLEMS There are two fundamental perceptions that are the basis of liner failures, and they are: 1. High density polyethylene (HDPE) is the only acceptable geomembrane material. This is wrong! 2. Geomembrane materials are commodity items and need only be purchased on the basis of cost. This is even more wrong! The acronym HDPE has become synonymous with geomembrane. Engineers assume it will perform in all applications and regulators approve it easily because other regulators approve it easily. In many cases regulations require its use, not by specifying HDPE directly, but by requiring a geomembrane with physical properties that only HDPE will meet. However, there are several conditions under which HDPE is the wrong material to use. In fact, not only are there other geomembrane materials with properties more suited to some applications, but there are many different HDPE products. Most HDPE geomembrane manufacturers make product using three or four

different resins based on availability and cost. And within the available resins there can be a factor of 500 difference in their long-term mechanical durabilities. Therefore, all HDPE geomembranes are not created equal. Engineers and regulators are not typically aware of this. Geomembranes are sold and bought as commodity items; consequently they are treated as commodity items. After all they are only thin sheets of plastic that conform to the profile of the subgrade and only need melting and pressing together to make a joint!! But should the geomembrane that is used for a hazardous waste liquid lagoon be made from the same resin, installed the same way, and have the same mechanical durability as the liner for a golf course pond? I think we would all agree that the former should be a far more durable material installed with higher quality. Thus, there are commodity installations and there are custom or specialty applications. Present geomembrane philosophy must be amended to recognize this, and to apply it in all sectors materials, design, installation, testing, and construction quality assurance (CQA). BASIC DESIGN PHILOSOPHY There are three fundamental liner designs single, double, and composite liners. Single geomembrane liners are typically used in non-critical applications. American philosophy is that a single liner cannot be installed without a few defects that leak. Thus, for more critical applications, a double lining system is required. Electrical leak location survey results on installed liners justify this philosophy. Naturally, there are the same number of leaks in the secondary (lower) liner as there are the primary (upper) liner, but if leakage through the primary liner is removed from the space between the two liners there is no hydrostatic head on the secondary liner. Therefore the lining system does not leak. Leakage will only occur if the leaking liquid passes over a defect in the secondary liner this is most unlikely. The double lining system is similar to double hulls on ships all ship hulls leak but the ship does not sink provided the leaking water is pumped out. If the leaking water is not pumped out from between the liners, the benefit and costs of a double lining system are totally lost. For the most critical applications composite lining systems consisting of a geomembrane and a clay or GCL liner are used. When there is a hole in the geomembrane the leak only acts on a small area of clay (GCL), provided there is intimate contact between geomembrane and clay. The permeability of a composite liner is about three orders of magnitude lower than either a geomembrane or clay liner alone. However, it is absolutely essential that the geomembrane and clay liner be in 100% contact. If there are wrinkles in the geomembrane or ruts in the clay the benefits of a composite liner are lost, and the additional cost is wasted. In the US, geomembranes are typically installed with some slackness (wrinkles) to allow for contraction in cold weather. The complete geomembrane layer is installed before the overlying layer is placed. This is acceptable if the liner is covered in the cold

weather, when the liner is flat and without stress, but invariably wrinkles are built into the geomembrane, losing the benefits of a composite system. In Germany, small areas of the complete lining system are built sequentially. At the end of the day the soil (drainage stone) that is to be spread on top of the geomembrane is placed in a berm around the edge of the liner to hold it in place. At night the geomembrane pulls tight to remove any wrinkles. Early the next morning, when the geomembrane is still cool and flat, the soil is spread. This ensures complete intimate contact, but with minimum stress in the geomembrane. The German approach is technically the better of the two but installation is slower and more costly. The concern about wrinkles is not only their impact on full contact between geomembrane and clay, but also relates to the effect of residual stress at creases and folds on the long-term performance of the geomembrane, particularly when the geomembrane material is HDPE. GEOMEMBRANE MATERIALS HDPE is considered the geomembrane material of choice due to its resistance to many chemicals and due to its high strength. However, in most cases, such as in municipal solid waste landfills, leachates are quite weak. Certainly there are concentrations of specific chemicals in the waste but by the time they have drained down to the geomembrane and have been diluted by infiltrating rainwater, they are no longer concentrated. In any case, many of these chemicals are detergents, chlorinated solvents, and oxidizing acids, which are in fact quite damaging to HDPE. It should also be noted that the strength of a geomembrane is really of little significance in liner design. No matter what the strength of the material, no thin geomembrane will support the weight of several meters of liquid or hundreds of meters of solid waste. The important parameter is the ability of the geomembrane to deform to accommodate any deflections or differential settlement in the subgrade and to deform such that the long-term performance of the geomembrane is not compromised. The philosophy of liner design should be that the geomembrane simply acts as a barrier and not as a load-bearing member of the lining system. This is extremely important for HDPE geomembranes due to HDPE s susceptibility to stress cracking a premature brittle fracture resulting from a constant stress lower than the yield or break stress of the geomembrane. The stress cracking susceptibility of HDPE is its Achilles heel and is essentially the only parameter that needs to be specified, since all other conventional mechanical and deformation properties of HDPE geomembranes are independent of the resin used. However the stress cracking resistance (SCR) of typical HDPE geomembranes can vary by a factor of 500 or more. Therefore, a grinding gouge at a stressed overheated seam that will cause failure in 1 year in a geomembrane made with an HDPE resin of low SCR will not cause failure for 500 years if a high SCR resin is used. Therefore, the low SCR material may be acceptable for a golf course pond liner but it should not be

used to contain hazardous waste liquids. This is an example of the differences between commodity and specialty applications. All HDPE s are susceptible to stress cracking (SC). It is a function of the crystallinity of the material, its molecular weight, its molecular structure, and the type of comonomer used. Figure 1 shows the applied stress/break time curve for five different HDPE geomembranes. At the higher stresses of the shallow slope the breaks are ductile, but as the stress decreases the slope increases and the breaks are all brittle. Therefore failure occurs much sooner than expected by extrapolation of the shallow part of the curve. Hence the requirement to install HDPE without stress in lining systems, particularly critical lining systems. Figure 1. Stress rupture curves for five HDPE geomembranes (Hsuan et al, 1992) Figure 2 shows that stress cracking is accelerated as temperature increases. Fortunately, in most lining systems, stresses decrease as liner temperatures increase.

Figure 2. Effect of temperature on stress rupture curve. Environmental Stress Cracking (ESC) occurs when SC is accelerated in certain chemical environments, such as detergents, chlorinated solvents, and oxidizing acids. For example black liquors at pulp mills and acids in mine solvent extraction facilities can be a problem. Thus it is very important when performing chemical resistance tests to do a test under stress to assess the potential for ESC. Most chemical resistance charts do not present the influence of stress. SC also becomes a problem when all the antioxidant (AO) additives in an HDPE geomembrane are consumed. Therefore, when sales people talk about HDPE having no additives similar to plasticizers in PVC they are correct on one hand - plasticizers are not needed to make HDPE flexible - but they are incorrect to imply that HDPE contains no additives that are consumed during service. When all the plasticizer is removed from PVC the material becomes brittle and cracks, but when all the antioxidant is removed from HDPE it also becomes brittle and stress cracks. The same thing will also happen with LLDPE and PP geomembranes when antioxidants and UV stabilizers are consumed. However, in most cases, sufficient additives are present to more than provide the expected service life. Thus, the oxidative induction time (OIT), a measure of the oxidation resistance of HDPE becomes important, especially where the geomembrane is not covered in service. In the OIT test a sample from the full thickness of the geomembrane is tested, so the test

results provide a performance parameter averaged over the full thickness of the geomembrane. Clearly, the exposed surface will be oxidized first and may lose its complete AO additive before the AO at some depth is affected. Therefore, the test data might show significant AO remaining in the test specimen even though the surface is completely depleted and susceptible to SC initiation. Therefore, the durability of an exposed HDPE geomembrane is a function of its OIT then its SCR. Thus, a material with a low OIT and high SCR may have the same long-term durability as one with a high OIT and low SCR. Clearly the high OIT and high SCR geomembrane is the one that should be used for the exposed primary liner of a double geomembrane liner for a hazardous waste liquid impoundment. In covered applications a high SCR is the most important parameter. However, there is one practical concern with OIT in the ASTM D3895 standard OIT test the specimen is heated at 200 o C to monitor the time to degradation. Since the AO package is made up of several different components providing protection over different temperature ranges the high temperature performance is not necessarily the same as would occur at lower operating temperatures in the range of 0 to 80 o C (geomembrane temperatures, not air temperatures). It would, therefore, be possible to have ample oxidation protection at operating temperatures but to have a low value of OIT. Of course, the opposite situation is also possible. Tests are presently being developed to take a thin layer (20 µm) of HDPE, representative of the thin surface layer of the geomembrane, to expose it to a temperature of about 85 o C (the maximum experienced in exposed service) for a few hours, and to measure the change in carbonyl group content a measure of the amount of oxidation. This more realistic oxidation parameter will then be combined with the SCR to provide an overall material durability factor (MDF) for exposed HDE geomembrane. The MDF can then be used to determine whether a specific HDPE geomembrane is more suited for commodity or specialty applications. GEOMEMBRANE MANUFACTURING There are essentially three methods of manufacturing geomembranes, two of which are used for HDPE. In both these methods all the components are mixed and driven through a screw extruder to feed the die. In one case the die is a wide flat slit, the widest of which is about 10 m. In fact, in this case there are two extruders feeding two conventional flat dies that are linked side by side. Flat extruders can control thickness to within about 3%. In the second case the screw feeds a circular die and a tube is extruded vertically. The tube is drawn (pulled) up about 25 m and is maintained stable by blowing cooling pressurized air into the tube. Thus, when the tube exits the die it is also increased in diameter somewhat. The thickness of the tube is controlled, not by changing the die gap, but by varying the tension and speed at which the tube is pulled upwards. The tube is flattened and taken over a roll at its maximum height, and is then brought downwards during which it is slit along one side, opened up, and then rolled up as a

single wide sheet. Consequently, these geomembranes have folds running along their length at the quarter and three quarter widths. Improper opening of the sheet after cutting can often lead to angled creases centered on the folds. These rolls are typically 7 m wide with thickness variations between 5 and 10% of nominal. Polypropylene geomembranes are made the same way. The third manufacturing method, calendering, is used for PVC and for some reinforced materials. The plastic mixture of all components is laid as a wide multi-layered ribbon between two side-by-side rollers through which it is rolled into a sheet, as shown in Figure 3. The sheet is guided around an inverted L-shaped bank of rolls including a final pair of rolls which generate the required thickness. The widest calendering rolls are about 3 m wide. The second pair of rolls may have a machined surface profile that is imprinted into the surface of the sheet a very fine mesh profile is known as a faille finish. Figure 3. Roll arrangements for calendering reinforced geomembrane The round die method of manufacturing sheet has the advantage that three extruder dies can be arranged concentrically so that three separate layers can be bonded

(laminated) together immediately after extrusion. This allows for several different modifications to the basic homogeneous geomembrane structure: 1. Chemically resistant HDPE on the outside and more flexible LLDPE on the inside 2. High SCR LLDPE on the outside and HDPE on the inside 3. A white surface layer to keep sheet temperatures low, which will minimize expansion wrinkling 4. Colored surfaces to suit the environment 5. Conductive lower surface layer to facilitate electrical leak detection as the final stage of construction quality assurance (CQA) 6. Textures on one or both surfaces White and colored surfaces preclude the use of carbon black us the UV stabilizer. Since carbon black is well established as the best UV stabilizer, specifying colored sheet automatically implies lesser UV resistance. Consequently only black sheet should be specified for projects requiring the longest exposed service. Textured geomembranes are made by blowing nitrogen into the mixture(s) in one or both outer screw extruders. As the material exits the die(s), it expands and explodes to the outer surface(s) of the sheet roughening the surface like the ocean. It is not easy to control the minimum thickness of geomembrane and the distribution and uniformity of the surface texture. The surface layer resin may be somewhat different (higher melt index) to the core resin in order to make it easier to texture. However care must be taken that this material does not compromise the performance of the complete geomembrane. Cadwallader (2001) has shown the use, in the surface layers, of what appears to be a recycled PE resin that is highly susceptible to stress cracking. Once the cracks have initiated in the surface layer they appear to propagate quite easily into the core geomembrane which, under normal circumstances, has a high resistance to stress cracking. Such products may not have adequate mechanical durability when used on slopes. Other methods of texturing one or both surfaces of an HDPE geomembrane include dropping an LLDPE powder onto the hot surface to which it melts and bonds, or by spraying the surface with hot molten fibrils of PE. However, in the past, small stress cracks have been noted to form around the edge of the particle where it is welded to the sheet, which will reduce the mechanical durability of the sheet, particularly if the texture causes a transfer of stress to the surface of the sheet. Thus, these textures require a compromise between minimizing the heating and thermal gradients to avoid stress cracking, and providing sufficient bonding so that the texture cannot be abraded off the surface. Once again, it is difficult to control the quality and distribution of the surface texture generated in these two methods. It appears to me that provided a resin with good SCR is used the nitrogen method probably produces the most SC resistant textured HDPE geomembrane. To provide a more uniform and consistent friction-enhancing surface, profiles can be generated in HDPE geomembranes by a calendering process immediately after extrusion. Such patterns include conical spikes, stubs, ridges, ridged cells, and swirly

profiles. Clearly a profile can be designed for friction increases with different interfaces. For instance, ridges may be appropriate for sand and clay surfaces, but they would not be very effective against woven geotextiles. A fine fibrous structure might work well against a nonwoven geotextile, but would not be very effective against sand. Therefore, all textured surfaces behave quite differently and it is absolutely essential to perform direct shear tests with the geosynthetic materials proposed for use and the actual site soils. Then, if randomly textured material is used, periodic conformance direct shear tests should be performed to ensure an adequate texture is being maintained on material delivered to the site. The use of textured surfaces on both sides of the geomembrane, particularly HDPE should be carefully considered. Since a design objective is to keep stress out of the geomembrane (particularly HDPE) it is important that the shear stress on the upper surface be lower than that on the lower surface. There are engineers who will only use a texture on the bottom surface and insist on the upper surface being smooth. In this way, if the layer on top of the geomembrane does move it will slide on the geomembrane and not tear it. The soil layers on top of the geomembrane can be reinforced with a geogrid or a high strength geotextile. GEOMEMBRANE SEAMS All geomembrane seams should be thermally bonded so they will not peel apart. It does not make sense to require that thermally bonded HDPE seams do not separate in a peel test, while allowing chemical seams in materials such as PVC to peel apart provided the peel force exceeds a minimum value. Seam stress cracking There is no question that a double track thermal fusion seam (set up correctly) is superior to a fillet extrusion seam. However, for maximum durability, the width and shape of the nip rolls in comparison to the width of the heating wedge should be such as to eliminate residual stresses at the root of the squeeze-out bead. Residual stresses can be identified by examining a thin-slice (10-15 µm) microsection under a transmitted light microscope with crossed polarizing filters; in HDPE residual stresses appear brightly colored. In susceptible HDPE geomembranes SC typically occurs in the lower sheet just under the edge of the weld bead of extrusion seams. It occurs at a shallow angle to perpendicular to the plane of the sheet, probably along the boundary between the isotropic melted and solidified weld zone and the oriented unmelted sheet material. If this is a sharp interface with rapid transition in microstructure it will act as a structural notch defect. However, if the transition is more gradual, such as might be developed by annealing, the SC susceptibility might be decreased. For this reason, it is believed that a combined hot air/hot wedge welder will provide the most durable seams the wedge does the melting and the hot air facilitates a more gradual temperature gradient at the edge of the wedge.

The most common location for stress cracks to occur is at and near the ends of extrusion beads, such as occur at a short bead repair or at the start/run-out of beads along seams at patches (Figure 4). It has been proposed that patches be placed on the underside of the liner so no run-out beads are necessary, or that the start and runout beads be done on top of the patch itself. Similarly, there are requirements that no more than two beads be placed in the same location to avoid overheating the geomembrane and increasing its susceptibility to SC. Clearly, the most trouble-free way to avoid SC is to use a geomembrane made with a high SCR HDPE resin so that it will accommodate the unavoidable abuse that occurs during installation. Figure 4. Typical stress crack at edge of extrusion bead

Seam testing When testing seams it has been shown that measuring shear and peel strengths provides no practically useful information on seam bond strength (Peggs 1996) [EG1]. Due to the relatively large seam shear area compared to the small cross sectional area of the sheet tab through which the seam is stressed, the sheet always breaks before the weld separates, provided the bond efficiency is more than about 8%. This figure varies between 8 and 20% depending on geomembrane thickness. This is quite evident when it is recognized that conventional allowable seam shear and peel strengths increase with geomembrane thickness despite the fact that what we are attempting to measure is totally independent of thickness! Therefore, if seam bond efficiency is only 25%, surely inadequate, we are not able to challenge the seam enough to be able to determine this. Therefore, there is little need to measure strengths. The important features of welding are to retain the ductility of the geomembrane adjacent to the seam and to ensure that the seam will not peel apart this is the best information we can get from the conventional tests. Ductility in the shear test is important to demonstrate that the geomembrane has not been mechanically damaged during preparation for welding and that it has not been overheated during welding. Separation in the peel test is important to demonstrate that a minimal degree of bonding has occurred. In HDPE, if peel separation can occur it is possible that crazing (Figure 5), the forerunner of stress cracking, is initiated in the separated surfaces. This can lead to significant reductions in the SCR of the remaining sheet. If this can occur in the laboratory it can occur in the field at wrinkles, at folds, where ice forms under flaps at the edges of seams, where subgrade slumping occurs, etc. Yet again, the best way to avoid these problems is to use a material with high SCR and that means higher than the conventional 200 hr in the ASTM D5397 notched constant tensile load test. This author has seen SC failures in material with a D5397 SCR of 240 hr. A minimum figure of 300 hr has recently been incorporated in the GRI-GMB specification. Clearly, the SCR should reflect the criticality of the installation.

Figure 5. Crazing induced in HDPE seam by peel separation Stress cracking tests can be performed on seams and textured material to confirm that the respective processes do not adversely affect the SCR of the basic geomembrane material. Clearly, a notch, as required in ASTM D5397 cannot be used, since this will result in testing the parent geomembrane at the root of the notch, bypassing the effect of the surface texture and of welding. Rather, a simple unnotched strip is used, with

carefully prepared undamaged edges, tested at 4MPa at 80 o C in a surface active agent (Thomas). Break time must exceed 700 hr. Occasionally, when peel testing HDPE, the welded interface does not separate but the geomembrane itself separates, as if delaminating (Peggs, 1985). This phenomenon, known as separation-in-plane (SIP), is seen more often in PP geomembrane seams (Figure 6). The SIP can travel part way across a weld track, across the air channel, and even beyond the second weld track. The cause of this phenomenon is presently being researched. It has been proposed due to incompatible carrier resins for the carbon black masterbatch and crystallinity gradients through the thickness of the sheet caused by differential cooling rates after extrusion. In extreme cases, it has been possible to delaminate both HDPE and PP geomembranes themselves, as shown in Figure 7. Therefore, it appears to be a basic geomembrane feature that becomes apparent when peel testing after welding. It is not a welding phenomenon. Figure 6. SIP in PP seam

Figure 7. Delamination in HDPE (Top) and PP (Bottom) geomembrane At present the Geosynthetic Research Institute is debating with its members whether SIP is an acceptable mode of peel break in any material, but at a minimum peel force. However, until it is clearly understood why SIP occurs, I feel that this is an unnecessary and retrograde step, since it is clearly possible to make both HDPE and PP seams that do not break in a SIP mode. Welding temperatures Project specifications often limit welding to certain ambient temperature ranges, or to temperatures measured a certain distance above the geomembrane. Neither of these temperatures has any significance. The only temperature that is important is the actual temperature of the geomembrane itself. This will be the ambient temperature when it is cloudy, and a much higher temperature when it is sunny; a black geomembrane can reach over 80 o C in summer sun. Typically PP geomembranes have a much wider welding window than HDPE and there are narrower gray areas at each side of this window PP is either welded or it is not. The weldability of HDPE is a function of the resin and its individual Melt Index. Hence the reason to specify that the welding rod for extrusion welds be made from the same resin as the geomembrane. However, this is simply a way of most easily assuring maximum weld quality, since an experienced installer can successfully join materials of

quite different Melt Indices. After all, LLDPE can be successfully joined to HDPE in coextruded geomembranes, and high molecular weight pipe can be butt-fused to lower molecular weight pipe. HDPE can be welded at temperatures below freezing and at temperatures above 75 o C, but this should be done with more frequent trial welds and seam tests. And trial welds should always be done in the environment and geometry in which production welding is to be done. For instance, trial welds should not be done in the back of a truck or on a smooth concrete surface when production welds are to be made on damp soils on a slope, or an a vertical wall. Textured sheet welding Care must be taken when welding textured sheets to ensure that they are clean. Damp soil or bentonite powder trapped in the texture will prevent effective welding. Consequently, many textured sheets are made with smooth edges to facilitate welding. However, when round die material is made with smooth edges, the smooth strip along the extruded tube is not as thick as the textured area so deforms more with the result that the edges of the sheet are longer than the center of the roll. Thus, the edges may be rippled (Figure 8), giving the impression that the quality of welding may be compromised. However, examination of such seams, when made by a competent welder, shows no problems with weld zone quality, weld symmetry and uniformity, and residual stresses.

Figure 8. Rippled edge of textured geomembrane There are a few continuing developments in joining HDPE and PP geomembranes separately and to each other. Electrofusion was initiated in 1987 (Butts) but is only now being developed by GSE for field use. The welding rod for extrusion welding is essentially extruded around resistance heating wires. A long length of wire is placed between the surfaces to be joined, they are ballasted, and power is applied to the heating wires, the rod melts, and welding occurs. In fact the method is more appropriate for patches where the rod can be prewelded to the patch, the patch placed over the defect, a weight placed on it, and current passed through the wires for a few seconds. Thus patching can be done by local labor. It is also claimed that such patches can be applied underwater, but this has not yet been adequately demonstrated. PE and PP geomembranes can now be given a surface fluorination treatment that enables them to be adhered to concrete, to themselves, and to each other with an epoxy adhesive. This again enables joining to be done by local labor rather than by costly professional welding crews. It also enables geomembranes to be fully bonded to vertical concrete walls rather than hanging them freely from batten strips at the tops of the walls. This improves liner durability and leakage performance. The treatment

also provides better barrier properties to hydrocarbons the gas tanks in BMW automobiles are made with a fluorinated HDPE. LINER PERFORMANCE EXPERIENCES We learn most about the performance of geomembranes and lining systems when they fail. We must make the most of these otherwise unfortunate happenings to extract most benefit from them. They should be investigated independently and lessons learned communicated to others so similar problems do not happen again. If there is a crack alongside a seam, an extrusion bead should not be applied to it; the crack should be cut out for possible future examination and the resulting hole patched. Examination of a fracture face can show where the crack started, the feature that caused it, and how it propagated. When a slope failure occurs and the liner is torn, the tear should be examined to determine if the tear was a result of the slide, or if the slide was initiated by the tear. This can be very important. The number of liner failures is increasing. My investigations find that most (~60%) are due to inadequate design, 20% are due to inadequate installation by installer or earthwork contractor, 10% are due to material deficiencies, and 10% are due to miscellaneous features. In most cases, the installer is held responsible. Often this is not justified. Conformance testing HDPE geomembrane was made in Europe and delivered to a site in South Africa. Conformance testing showed the material to meet all project specifications except SCR; break time in the ASTM D5397 test was 60 hr instead of the specified minimum of 400 hr. The manufacturer insisted the material was acceptable, but he had not tested it. The resin manufacturer insisted the SCR had been measured to be about 360 hr. The material was rejected and new material had to be made in the USA. The specification was easily met. It should be noted that the ASTM D1928 standard procedure for casting plaques on which to do resin tests allows three cooling rates. Each of these rates will generate different amounts of crystallinity in HDPE. If the cooling rate was too fast the crystallinity would be lower than in a geomembrane and the SCR would be unrealistically high. In the owner s next project HDPE was being made in Korea, but the manufacturer could not make all the material in time. Some material was ordered from Europe. This time conformance testing was done before material was shipped to South Africa. The Korean material met specifications, the European material did not meet the OIT specification of 100 minutes at 200 o C. Test results showed a value of 101 minutes but the test was performed at 180 o C. At 200 o C the result was found to be about 75 minutes. According to ASTM D3895 the test can be performed at 180 o C, if specified, but if no temperature is specified it must be done at 200 o C. The engineer was prepared to reject this material, but the CQA consultant thought this was unnecessary since the material was not be used exposed. Subsequently the material was accepted

for use. In addition, the engineer was not willing to support the generation of data to support this decision, but required that the installer/manufacturer support the CQA consultant in generating the information. This is not standard CQA procedure as usually contracted by the owner to the engineer the engineer s responsibility is to ensure that the owner gets a properly performing installation at the best cost, and that includes properly dealing with nonconforming materials. Exposed liner cracking An HDPE geomembrane was installed in a landfill cell in Florida two steep slopes were left exposed. After 8 years, cracking occurred in seams, at the side of seams, and along apex-down folds just below the tops of the slopes. Apex-up folds did not crack. There was also cracking at a protrusion underneath a patch (Figure 9). Despite the fact that the SCR of the material was about 240 hr and the OIT just over 100 min, both meeting project specifications, the surface of the material had oxidized and cracks had initiated on the surface. The stresses required to initiate the cracks were contraction stresses (even though temperature changes were relatively small) and wind induced stresses. Only apex-down folds had cracked because the upper oxidized surface was in tension when the sheet contracted. In the apex-up folds the oxidized surface was in compression. Yet again, if this material had had a higher SCR this cracking probably would not have occurred.

Figure 9. Cracking (Top) at protrusion under HDPE geomembrane patch (Bottom) Cast-in liner chemical resistance An HDPE cast-in liner, about 5 mm thick, was specified for use on the walls in several concrete basins of a heap leach mine solvent extraction system. Loose geomembrane was used to line the floors. There were no specifications for SCR or OIT. The specifications required an HDPE that was compatible with a list of liquids at specified temperatures. The settler unit contained mixtures of kerosene, naphtha, oxidizing organics, and sulphuric acid. The engineer was urged to do chemical resistance tests but declined to do so, arguing that such a system had worked successfully before so there should be no problems. This was despite the fact that some chemical resistance tables show incompatibility between PE and naphtha and kerosene at temperatures close to the maximum specified. Large gaps between cast-in sheets on the walls that could not be extrusion welded were capped with loose sheet and welded. Spark tests were performed on all welds and repairs were made. The basin was filled with water. Leaks were found. The basin was emptied. Spark testing was performed and additional leaks were located. The engineer thought the installer had done an inadequate leak survey the first time. However, leaks were now full of water and were more conductive so it was not surprising that more were found. The facility was filled with organic solution once again more leaks were found. Again the engineer was dissatisfied with the installer, but the organics were less viscous than water so would penetrate even finer passages.

Finally the acid (including copper) was added and the temperature raised to the service temperature - then about 20 o C higher. Leaks started to occur as welds along the edges of the cap strips over the joints started to separate. Welds at corners started to separate, and at any location where an unsupported sheet was welded to a supported sheet. The general contractor and liner installer were soundly faulted for a bad installation and poor welding. The unsupported sheet was absorbing organics and swelling. The cap strips were bowing and inducing a peel stress on every millimetre of weld. When seams are destructively tested it is not unusual to allow one peel test of the five peel and five shear tests to fail while still accepting the complete weld sample. This might be acceptable when, as should be designed, the liner is not subject to stress. However, a one in five failure rate is a 20% failure rate! Therefore, it might not be surprising that when all of the seam is subjected to a peeling stress in service the most poorly bonded segment within the unacceptable 20% will fail. Then, when it is repaired, the next weakest segment will fail. And so on! But repairs also become more difficult when the surface of the sheet is saturated with organics and the owner wants to keep down time to a minimum. Sufficient time must be allowed for the organics to volatilize or be removed, if effective repairs are to be achieved. The next problem is that the swelling and rippling stresses build up adjacent to seams, where the liner has been heated and more of the antioxidants have been consumed. The sulphuric acid and the oxidizing organics, particularly at operating temperatures 20 o C higher than the already high operating temperature, cause additional oxidation at the surface in those regions and stress cracks are initiated. Once initiated, stress cracks apparently propagate quite easily. And it has already been described how stress cracking is accelerated at higher temperatures. Therefore, cracking of the unsupported liner occurs in and adjacent to the welds. Clearly, the appearance was of a major welding problem. However, the primary cause of the problem was not the HDPE or the welding, but the insistence of the engineers in using HDPE in an application that was totally inappropriate for HDPE. The engineer was obviously not aware of the performance characteristics of HDPE and was not prepared to take the advice of those who did know. An assumption was made that temperatures could be raised higher than those in previous installations without any changes to the design, and then temperatures were increased even further. If the engineers were not aware of the performance characteristics of the liner, the regulating agency engineers would know even less. Clearly the regulators would rely on the integrity of the engineers presentations a big mistake. These swelling problems occurred at at least two mine sites, and cost many hundreds of thousands of dollars in down time, repair time, lost production, testing, consultants, and resulting legal fees. All essentially because a $10,000 chemical resistance test was not performed. One of the more significant results of this exercise is that geomembranes unjustifiably lost many supporters the HDPE liners were replaced by fiberglass.

Leaking cast-in liner A similar thing happened when a double lining system was placed inside a large concrete basin of a major multinational corporation. When the basin was filled with water the primary liner leakage collection system was dripping not a constant stream, but just dripping. The owner requested the installer to make repairs since the liner was not leak-free. Repairs were made but then the leakage collection system was constantly running. After a second set of repairs the leakage rate was even higher. The owner did not understand the philosophy of the double lining system and that all single liners should be assumed to leak. How difficult would it have been to collect the few initial drips and recirculate them into the basin!? Again, due to unrealistic expectations, and totally unnecessarily, geomembranes fell into disfavor the lined concrete basin was replaced with a stainless steel tank, and at much greater cost. Geosynthetic clay liners Here are two examples of geosynthetic clay liners (GCL). In a wastewater treatment plant lagoon a GCL was specified for use by the engineer. His initial calculations using the manufacturer s specified hydraulic conductivity (HC) showed that the seepage rate would not meet the State s maximum allowable value. He asked the manufacturer if the calculations were correct the manufacturer said they were. The engineer then found an older specification with an HC value 20% of the present value. He asked the manufacturer if the lower value could be guaranteed the manufacturer said No. The desperate engineer asked what would be the lowest guaranteed value the manufacturer provided a number and still this was not good enough. The manufacturer suggested using a GCL incorporating a PE film. The engineer declined The engineer then remembered that at the wet time of the year the groundwater level would be higher than the floor of the pond thereby reducing the head on the liner. This would have been true in the case of a geomembrane but not in the case of the GCL. With the lower head and the minimum guaranteed HC the seepage rate would just meet the State s maximum allowable value. The pond was built, with very careful records maintained by the general contractor. On completion, and with a 450 mm ballast layer on the GCL, the pond was to be filled to a depth of about 3.5 m for a fullscale hydrostatic test. At just over 2 m the pond was leaking faster than it could be filled! Not only was the GCL basically ineffective, the stones in the soil layers on each side of the GCL were as large as 300 mm, compared to the maximum recommended by the manufacturer of 25 mm, and the fine fraction in the soil was only 15% the volume recommended by the manufacturer for uniform pressure confinement of the GCL. When surveyed and uncovered there were 55 groups of holes in the 1.25 ha GCL. And the general contractor and manufacturer were blamed for a low quality GCL badly installed. Ultimately the engineer was found responsible, and the GCL was replaced with a geomembrane as should have been used in the first case. In the second case, an unballasted GCL was used to line a decorative pond. A sharp rock aggregate with about 10% of fines (rather than the recommended minimum of 80%) was used as the subgrade the sharp edges had torn the lower geotextile of the

GCL. Batten strips were used to fasten the GCL to peripheral walls, but in many locations sections of batten strip were missing. Sharp limestone rocks were placed on the GCL around the periphery of the pond they had punctured the GCL and were providing calcium ions for cation exchange with the sodium ions in the GCL, thereby reducing its HC. Transition of the peripheral GCL from batten strips on the walls to natural materials confinement behind the rocks and soil were not sealed, and were loose enough to push one s arm between the GCL and the end of the wall. This 750 m 2 pond was leaking 136 m 3 of water each day. This was a poor design that was badly installed. The owner paid for the investigation, re-engineering, and installation of a new lining system. Clearly the concept of commodity and specialty applications for geomembranes also applies to every sector of the industry there are engineers, installers, contractors, laboratories, and CQA firms who are only capable of providing commodity services and there are fewer who are capable of providing the knowledgeable services required for more critical and custom applications of geomembranes, GCLs, and lining systems. QUALITY PROGRAMS The first stage of a quality program is to ensure that those involved with the project are experienced and knowledgeable in working with geosynthetics. Clearly this is not as important for a golf course pond as it is for a hazardous liquid lagoon or a liner on a steep quarry wall, although it is important to the golf course owner! For those working with HDPE it is important that they understand the stress cracking phenomenon. For exposed HDPE the OIT parameter must be understood. For PP, contractors must be aware of the potential for SIP. For PVC, contractors must be aware of the capabilities if chemical and thermal seaming and of the significance of plasticizer content. For GCLs, an understanding of the reason for a uniform and adequate confining pressure is essential. The initial difficult part of this, of course, is ensuring that the engineer, who knows more than the owner and regulator does, in fact, understand what is being said. The only way to confirm this is to have proposals, drawings, and documents submitted for peer review by an established expert. Then a quality assurance program becomes quite routine. However, it is most important to realize the differences between quality control (QC) and quality assurance (QA). QC is a quality program established by a contractor to ensure the quality of its own work. Therefore, all parties to a project will have their individual QC programs. QA is a quality program established typically by an owner to ensure that the work done by others is done according to the project specifications and related documents. A QA program, therefore, does not ensure that a perfect structure is built if a poor design is adopted, the QA program will ensure that that poor design is built. The most effective QA (only available from experts) will identify problem areas before construction commences and attempt to have them resolved. It will also ensure that, within the constraints of the budget and schedule, that work that is done will be to the highest standard. Clearly, if the installer is not an experienced installer, that standard will not be as high as it would be for a highly experienced installer. Such standards are

established by the budget for the project, and the amount the owner is willing to pay. If the owners simply want the lowest cost, they will get the lowest quality. The objective of liner construction QA (CQA) is not only to ensure that what was designed is built, but if there is a problem with the liner in service that construction records are adequately comprehensive to enable a full analysis of the construction process to quickly determine the full extent of the problem. For instance, if a weld separated, how many seams did the welding machine and/or the operator make between passing trial welds? If it is a material problem, where was the rest of that roll used? Therefore, the best CQA personnel are those that have previously been welders they know what can be done and what can t be done. They also know when quality can easily be improved. Typically, an engineer does not make a good CQA monitor the job is not sufficiently creative. And an inexperienced CQA monitor that simply annoys a good installation crew will be an absolute waste of money and likely result in a lower quality installation than if the installer had been left alone. To this end a monitoring program based on the following should be implemented: 1. Review selection of geomembrane material for the project 2. Review specification for material 3. Review project specifications and drawings 4. Review CQA plan and ensure no gaps or conflicting overlaps with other project documents, including contractors QC programs. 5. Ensure CQA plan describes actions in the event of non-conformance. 6. Review resin manufacturer s, geomembrane manufacturer s, and installer s QC certificates and programs to ensure there is full traceability of materials, people, and equipment used, in the event of construction and in-service problems 7. Ensure manufacturer and installer understand and can comply with all project documents 8. Confirm that all materials arriving on site meet project specifications. When materials are manufactured overseas it is important to do this before material leaves the manufacturing plant. Unfortunately, it is not sufficient to rely on QC certificates. 9. Review proposed panel layout diagrams and liner penetration details 10. Ensure subgrade is suitable for placement of geosynthetics 11. Monitor and record details of all on-site construction activities 12. Identify and record all roll, panel, and seam numbers 13. Ensure trial welding is performed under the conditions of production welding 14. Ensure all failing trial seam results are recorded 15. Monitor all welding 16. Monitor all seam nondestructive and destructive testing 17. Select samples for destructive testing at the independent laboratory and review test results. 18. Monitor numbering and testing of all repairs 19. Prepare panel layout drawing 20. Monitor placement of first soil layer on top of geosynthetics this is when most damage to the geomembrane occurs.. 21. Perform a geoelectric integrity survey over the complete liner surface. 22. Record all meetings in which problems are identified and resolutions proposed

23. Prepare comprehensive final report including copies of all QC certificates, deployment logs, welding records, test equipment and results, design modifications, etc. If CQA is professionally performed, responsible general contractors and installers will not view CQA as an imposition or a nuisance, but as an unbiased means of helping them achieve the highest quality and most durable lining system. Under no circumstances can effective CQA (on behalf of the owner) be performed by the general contractor or the installer. LINER TESTING Conventional testing procedures during liner installation include seam destructive (peel and shear) testing, and seam air pressure, vacuum box, and spark testing. Obviously it is most undesirable to cut sample of good double wedge welds out of the seams for destructive testing then to make repairs with three times the length of an inferior extrusion weld. The majority of the liner is not tested other than by cursory visual inspection. This can be singularly ineffective, as outlined below. The newly installed liner of a concrete basin in a wastewater treatment plant was leaking. The owner spent nine months raising and lowering water levels, using dyes, and making visual inspections but could not locate the leak, nor even the level at which the leak occurred. Within four hours a geoelectric survey found a pinhole leak in a weld at a floor/wall corner and two parallel holes approximately 20 mm long and 2 mm wide about 15 mm apart on the floor caused by a dropped tool. These holes were very clear visually after they had been located, but they had not been found during many previous visual inspections. Geoelectric leak and integrity surveys Geoelectric surveys have been commercially routinely performed since the mid-1980s. The general principle of the technique is based on the geomembrane being an electric insulator. A potential is applied between an electrode in the water or soil/waste above the geomembrane and another in the soil or leakage collection system below the geomembrane. In the ideal situation current will only flow through the leaks we are trying to find. This assumes that pipe penetrations through the geomembrane, soil around the edges, or rainwater wetting the edges of the liner do not also provide pathways for current flow. A survey probe is then use to measure the potential gradients between the electrodes of the search probe in the water or soil above the geomembrane. Where there is a leak and a high current density there are also higher than background potential gradients. Thus the leaks can be pinpointed. Variations of the technique allow liquid-covered liners (any depth), uncovered liners, and soilcovered liners to be surveyed. I have outlined (Peggs, 1999) design features that should be considered if effective geoelectric leak/integrity surveys are likely to be required. By wading in shallow water holes of 1 mm or less in diameter can be exactly located. In deep water, which requires dragging a probe from one side of the pond to the other,

the same size of hole can be identified but location is typically to within 500 mm. A liner covered by 1 m of sand will have a 3 mm diameter hole located to within 150 mm. Successful surveys have been performed on 640,000 m 2 of uncovered LLDPE geomembrane, and on geomembranes under 5.5 m of heap leach ore (25 mm hole located), 5 m of municipal solid waste, and 18 m of industrial waste (Peggs and McEuen, 2001). In addition to these portable techniques there are several techniques for installing a series of electrodes underneath the liner during construction that will subsequently continuously monitor a lining system for leaks, indicating when a leak occurs and approximately where (Nosko, de Meerleer, Arndt). Usually a portable method is required to identify the exact location of a leak. General experience shows that only occasionally do leaks develop after the liner is placed in service. Leak statistics Nosko and colleagues (Nosko et al. 1996, Nosko and Touze-Foltz, 2000) have presented some very useful statistics of liner damage that occurs during construction, which shows where the emphasis should be during CQA. Table 1 shows that most damage occurs during placement of the soil layer over the geosynthetics. Most damage is caused by stones and heavy equipment. Therefore, in the case of landfills it is essential the CQA program does not finish when the geomembrane is installed but continues as the geosynthetics are covered. However, in uncovered pond liners, most of the damage is at seams. Table 1. Type of damage and when it occurs When Installation Covering After Covering Type 24% 73% 2% Seaming 79% Stones 17% 60% Cuts 4% Stakes 16% Equipment 16% 64% Components 27% Weather 9% Table 2 shows where most of the damage occurs most is on the floor, not at corners or penetrations where it might be expected to be. Then Tables 3 through 7 show the types and frequency of damage at each of these locations.

Table 2. Location of damage Amount of Damage Flat Floor Corner edge Under a drainage pipe Pipe Penetrations Other 4194 3261 395 165 84 289 100% 77.8% 9.4% 3.9 % 2.0% 6.9% Table 3. Type and frequency of damage on floor Type of failure Number of holes % Stones 2641 81.00 Heavy equipment 430 13.20 Worker 130 4.00 Cuts 33 1.00 Welds 26 0.80 Total 3261 100.00 Table 4. Type and frequency of damage at corners/edges Type of failure Number of holes % Stones 234 59.20 Heavy equipment 75 18.90 Worker 14 3.50 Cuts 4 0.90 Welds 69 17.50 Total 395 100.00 Table 5. Type and frequency of damage under drainage pipes Type of failure Number of holes % Stones 50 30.30 Heavy equipment 24 14.30 Worker 24 14.50 Cuts 23 13.70 Welds 45 27.20 Total 165 100.00 Table 6. Type and frequency of damage at pipe penetrations Type of failure Number of holes % Stones - - Heavy equipment - - Worker 7 8.50

Cuts 1 0.60 Welds 77 90.90 Total 84 100.00 Table 7. Type and frequency of damage at other locations (road access, temp. storage, concrete structure, etc.) Type of failure Number of holes % Stones 60 20.60 Heavy equipment 125 43.40 Worker 56 19.30 Cuts - 0.00 Welds 48 16.70 Total 289 100.00 Similar statistics for a 640,000 m 2 single liner placed on a soft wet peat subgrade are shown in Table 8. There was an average of 2 holes/ha in this liner that was subjected to comprehensive effective CQA. However, it should be noted that that the leak survey was performed as construction progressed so all holes identified by the CQA team had not been repaired prior to performing the survey. Table 8. Large single liner leak statistics re. types and sizes of holes. Size (mm) Punctures Gouges Cut Tears Burns Scrapes Lack Seam s Of bond < 1 10 1 2 1 1 1 2 10 28 1 8 7 4 1 11 50 7 11 7 2 3 2 1 51 100 1 3 1 1 3 101-500 1 1 1 1 501-1 m 1 2 > 1 m 2 1 1 Unknown 4 3 1 2 1 2 Total 50 16 13 5 8 17 10 12 % total 38.2 12.2 9.9 3.8 6.1 13 7.6 9.2 Infrared spectrometer surveys Electrical surveys can be performed at the rate of about 1 ha/day but they are not particularly suited for doing landfill caps. However, if there are leaks in caps they will be emitting landfill gas (LFG). A portable multichannel analyzer, effectively an infrared spectrometer, has been developed that will analyze methane, carbon dioxide, and

total hydrocarbons several times every second. This equipment was used to sample LFG from about 150 mm above a 6 ha landfill cap surface while being carried in a truck at about 15 km/hr. Location was monitored by global positioning system equipment. The complete site was monitored on parallel tracks approximately 1.75 m apart as shown in Figure 10 in 1.5 days. This included identifying over 30 leaks then searching each of these leaks for the exact location of the maximum concentration. It is estimated that this technique could survey up to 150 ha per day, depending on topography. Figure 10. Landfill cap leak survey record This technology has also been used to locate the source of actual cap leaks on slopes and near culverts and roads when the surface gas concentration peak could be some distance form the actual leak. It was found that surface gas concentrations could be very localized (to within 50 mm) and that the actual leak could be several metres away from the maximum surface gas concentration. Development of this technology is underway to apply it to the evaluation of newlyinstalled bottom liners, both with and without soil covers, and to large evaporation pond and heap leach pad liners - the survey time could be decreased to 1% of that required for geoelectric surveys.

Ultrasonic and infrared thermography seam testing Two methods, ultrasonics and infrared thermography, have been proposed for the nondestructive evaluation of geomembrane seam bond strength, in order to avoid cutting holes in seams for destructive peel and shear tests. Ultrasonic measurements of seam thickness were attempted and terminated many years ago by Schlegel Lining Technology. They have recently been revived in Region 2 of the United States Environmental Protection Agency (USEPA). A pulse-echo technique is used to essentially measure the thickness of the completed seam. Based on German geomembrane seam analyses (Lüders, 1996) the USEPA rationale is that if seam thickness is between 0.3 and 0.8 mm less than twice the thickness of the geomembrane, sufficient melting and mixing at the interface has occurred and the seam is acceptable. Measurements are made every 8 m along the length of the seam, a considerable improvement on the 150 m sampling frequency for destructive tests. However, it is apparent that a reduction in seam thickness can be achieved in two ways by high pressure and low temperature, or by high temperature and low pressure. It is doubtful that both methods generate the same quality of weld bonding. Lüders (2000) clearly elaborates that the thickness reduction is a secondary effect of requiring a minimum depth of melt zone at the interface. Therefore, there must also be some evaluation of weld zone morphology before finished seam thickness alone can meaningfully be used as an acceptance criterion. The transducers used for ultrasonic testing are conventional flat-ended units that require a couplant to transfer the signal between transducer and geomembrane. This is time consuming, requires that seams be supported by the subgrade, and means that extrusion seams, with their rough surface profiles, cannot be tested. Extrusion seams are of a lower quality than double wedge seams so are more important to test. Noncontact Airscan ultrasonic transducers are now available that can be traversed along a seam with a transmitting transducer inducing a signal in the geomembrane on one side of the seam and a receiving transducer monitoring changes in the signal that is picked up from the other sheet after the signal has passed through the weld interface. Thus every millimetre of seam, even on wrinkles and pipe boots, can be tested - extrusion seams as well. However, infrared thermography (IRT) offers a more rapid and direct method of evaluating seam bond strength and internal flaws nondestructively (Peggs et al, 1996). This method requires the surface of the weld to be heated about 10 o C and then to monitor the surface temperature a second or two later as heat is conducted into the geomembrane. A thermogram is shown in Figure 11. Heat is rapidly absorbed at good seams (blue) and is not absorbed where the bonding is poor (red). Voids and grains of sand in the seam can clearly be seen, as can blockages in the center air channel. Even changes caused by the thermal cycling of the hot wedge can be identified. This is far more effective than performing peel and shear tests.

Figure 11. IR thermogram of double track seam. With appropriate artificial Intelligence (AI) it is projected that seams could be surveyed at about 10 km/hr, with flaws being analyzed in real time and with the location and type of critical flaws being identified by a spot of spray paint on the liner. A hard copy (videotape) of every millimetre of the seam would be available for further examination. Such technology is available now for special investigations, but needs the development of the AI for routine CQA purposes. GEOMEMBRANE LINER LIFETIME In general geomembrane materials of all types are providing very satisfactory service, but we still do not have much more than 40 years experience with PVC, 25 years with HDPE and 8 years with PP and GCLs. However, despite this tremendous success, there have been liner installations that have not lasted 8 years or even 6 months, and there are some that have not worked on commissioning. These are typically due to a lack of knowledge by at least one party to the project. This is, unfortunately, to be expected in a new, growing industry, in which those with appropriate knowledge do not have the resources to educate the large number of people entering the industry. Thus, while the level of knowledge is increasing at the leading edge, the overall level of knowledge of those involved in the complete industry may, in fact, be decreasing! However, if a geomembrane-based lining system is properly designed, the correct material selected, the material (and its grade) is properly specified, it is properly installed, knowledgeable CQA is performed, it is properly tested, and the facility is